Bone sialoprotein (also called bone sialoprotein II, BSP) is a major non- collagenous protein in bone and other mineralized connective tissues. BSP expression is essentially restricted to bone- and dentin-forming cells and maximal expression correlates with the initial formation of mineralizing matrix. Although it is suggested that BSP has an important role in bone formation and mineralization, the precise function of this relatively, newly-discovered protein has yet to be determined. The long-term objective of the application is to study the mechanisms of regulation of rat BSP gene expression in bone and tooth tissues. The specific aims proposed are to: I) determine the region of the rat BSP promoter that confers the tissue-specific expression of BSP during embryonic and neonatal development in transgenic mice; determine the effects of glucocorticoids (dexamethasone) and 1,25-dihydroxyvitamin Do [1,25-(OH)2D3] on the expression of the rat BSP transgene; 3) examine the alterations in BSP expression in vitamin D-deficient rickets and to relate these changes to defects in bone formation, mineralization and remodeling. To achieve these goals, transgenic mouse lines harboring a 2.7 kb rat BSP- 5-prime-flanking regulatory element and three deletion mutants fused with fire fly luciferase (LUC) gene will be produced. LUC assays, Northern and in situ hybridizations will be performed to define the minimal DNA sequence in rat BSP promoter region that controls tissue-specific expression. Transgenic mice will also be administered dexamethasone and I,25-(OH)2D3, respectively, followed by quantitative analyses of Luc mRNA level to identify the promoter constructs that contain the sequence elements responding to the hormone stimuli. Finally, a rat rickets model will be utilized to analyze the altered expression of BSP gene and the synthesis and secretion of BSP protein in a pathological condition. Results from these studies will provide important insights into understanding of bone and tooth development as well as diseases of mineralized tissues, such as periodontitis and osteoporosis that afflict millions of Americans. Information about steroid regulation on the expression of bone proteins may lead to improved understanding of disorders such as rickets and to practical application in the treatment of such diseases.